Current supervision circuit

ABSTRACT

A circuit for supervising an electric current and giving an alarm when the current exceeds a predetermined upper limit defined by a first threshold detector, or drops below lower limit defined by a second threshold detector.

United States Patent [72] Inventor Arne Lennart Svansson [56] Referen e Cited z v Swede UNITED STATES PATENTS a 3 1 1967 2,585,589 2/1952 Rockafellow 340/248 1 e y 2,946,010 7/1960 Tarczy-Homoch 328/1 16 [45] Patented Sept. 28, 1971 3,214,700 10/1965 Hook 340/253 [73] Asslgnee Aktiebolaget Atomenergl 3 281 610 10 1966 B l 307 235 Stockholm Sweden 3 2 11 101' 8! a 3,305,787 2/1967 Distler et al. 307/235 3,346,846 10/1967 Ferguson et al. 307/235 1 CURPENT SUPERVIQON CIRCUIT Primary Examiner-Alvin H. Waring 7 Claims, 2 Drawlng Flgs- Assistant ExaminerDaniel Myer 52 us. Cl 340/253, Atwrnekliercet Scheffler & Parker 250/831 [51] Int.Cl G08b 21/00, 8

GOlt 3/00, H01 j 39/33 ABSTRACT: A circuit for supervising an electric current and [501 Field of Search ..340/248-256, giving an alarm when the current exceeds a predetermined 228, 412-415; 250/83, 83.1, 83.3, upper limit defined by a first threshold detector, or drops 83.6; 307/235; 328/115-117, 146-149 below lower limit defined by a second threshold detector.

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Arne Lenncuk svmnss CURRENT SUPERVISION CIRCUIT I The present invention is concerned with a current supervision circuit for supervising possible excursions of a current from a normally present, and between a lower limit and an upper limit situated, and preferably substantially constant current value for providing supervision alarm when said current value exceeds said upper limit during a continuous first time period. The invention may be used for detecting, for example, a neutron flux, especially such a short and intensive neutron flux which may be created at a nuclear accident.

At the treatment of some fissionable materials like Pu-239 and U-235 there exists a risk that a critical system is created and that a so-called nuclear excursion will occur. Such an excursion may under certain circumstances give rise to a real explosion, but for most of the so far reported incidents, the only obvious sign that an excursion has occurred is a short but extremely strong radiation intensity from the system. Since persons staying in the vicinity of the place for the excursion then may receive a dangerous exposure of radiation, it is of utmost importance to get an alarm immediately.

Data based upon earlier occurred nonexpected excursions shows that a criticality accident will yield totally from l to fissions with a duration which may vary from microseconds to minutes. The excursion is characterized by the burst of neutrons which always are emitted from the system. This burst has always an easily identifiable intensity peak with a very short duration from microseconds to milliseconds, often followed by a longer intensity plateau on which is included the'main part of the total amount of the fission neutrons. There is always a big risk thatthe nuclear excursion described above may repeat-itself several times.

Where a risk for a criticality accident exists, a selective monitoring and alarm system is required which is able to give information and alarm for every excursion which may occur independently of its duration or its magnitude. Since such an alarm must give rise to a complete evacuation of all people in the actual area, one must be absolutely sure that the number of false alarms is reduced to an absolute minimum and that the reliability of the monitoring system is as high as possible.

In order to be able to detect with certainty an excursion of extremely short duration, it is well known that it is possible to use a dose-measuring instrument which is based upon the application of neutron or gammasensitive detectors which are combined with some forms of data-handling units performing integrating functions. The reliability of the detectors, the necessity to use threshold limits to avoid integration of the background intensity and limitation in measuring ranges are factors which are difficult to overcome without making the detection system complicated and difficult to manage.

The invention has as its object to provide a current supervision circuit fed by, for example, a detection device with a detector which itself has the feature of performing an integration of the dose rate from 'a criticality excursion. Said detector, however, forms no part of the present invention.

The current supervision circuit, according to the invention, is characterized by the fact that the circuit comprises a first threshold detector and a second threshold detector, each said detector having as an input signal one and the same signal corresponding to the value of the supervised current, with said first threshold detector arranged to break the input signal to as well the second threshold detector as its own input terminal when the current exceeds said upper limit, and to for a predetermined second time period provide a change in its own working condition which brings about that a renewed supply to said first threshold detector within said second predetermined time period of an input signal corresponding to a current exceeding the upper limit will not result in a repeated breaking of said input signals; and, further, to within said predetermined second time period provide a renewed supply of the input signals to both threshold detectors, and with said second threshold detector arranged for providing for an input signal corresponding to a current value below said lower limit, a changed condition in an auxiliary circuit, which is also influenceable by said first threshold detector, during substantially said second predetermined time period, so that at the renewed supply of the input signals to both threshold detectors and when the input to said first threshold detector corresponds to a current value exceeding said upper limit, the influencing of said auxiliary circuit by said first threshold detector is arranged to result in the triggering of an alann circuit.

The invention will be described in connection with the attached drawings:

FIG. 1 shows an embodiment of a current supervision circuit according to the present invention, combined with a neutron detector which supplies a current to be supervised by the current supervision circuit.

FIG. 2 is a graph showing the variations in the current delivered by an amplifier forming part of the circuit.

The neutron detector, which is shown in sections in FIG. 1, is of the beta-current-type. lt comprises an emitter in the shape of a rectangular plate 10 which preferably is composed of Rh 103 or Ag 109. The emitter is surrounded by an isolator 11 which preferably has a layer of polystyrene, polyethylene or-for high temperatures-aluminiumoxide. The insulator is surrounded by an envelope 12 which serves as a collector and which may consist of stainless steel. To the emitter is a hollow cylinder 13 fixed, made of stainless steel. The cylindercontains a radioactive beta particle emitting source, preferably Sr 90. To the detector is connected a coaxial cable, the outer conductor 15 of which is connected to the collector l2 and whose inner conductor 14 is connected to the emitter 10. Between the conductors there is an insulating layer 16, preferably of polyethylene or aluminiumoxide.

The function of the detector is as follows. The radioactive source in the cylinder 13 is continuously generating a current which is passing through the coaxial cable. This current serves as a check current. When the detector is exposed to neutron radiation, radioactive isotopes will be generated in the emitter I0, viz, Rh I04 if the emitter contains Rh 103 and Ag l 10 if the emitter contains Ag 109. These radioactive isotopes are short-lived with half-lives of the magnitude of a minute. They are decaying, emitting beta radiation which is generating an electric current proportional to the neutron flux. In the coaxial cable this current is added to the check current.

As may be seen from FIG. 1, the above-described beta-current detector is connected to a linear amplifier 3 via an electronic gate 2. This amplifier has been designed in such a way that it can accept an input signal from IO A. to l0'A. without range switching. The output signal from the amplifier 3 gives to two threshold units or detectors, viz, a first threshold unit or detector 4 which will respond when the signal exceeds a preset value and a second threshold unit or detector 5 which will respond when the signal falls below a preset value. From the threshold units the signal willif it it above or below the earlier mentioned threshold values-pass to the actuation circuit 6. This circuit is arranged to give an alarm to alarm unit 7 if it receives signals from both threshold units. Further, the actuation circuit 6 is arranged to check the received information and to delay the outgoing alarm command until this checking has been made.

The device according to FIG. 1 comprises further on a test pulse generator 85, the function of which will be described later on. The device comprises also a power supply 8 which is arranged to deliver a constant supply voltage to the units 2, 3, 6 and 7. To the power supply unit 8 is connected an instrument alarm unit 86. This is continuously checking the current delivered by the power supply. Should this current change noticeably, there is an indication that a fault has occurred in the system and unit 86 will therefore give alarm, i.e., an alarm which is different from the alarm given by alarm unit 7.

If there is for example an open circuit in the coaxial cable 14-16, the second threshold unit 5 will not receive any input signal. The unit 5 will then give an output signal to the actuation circuit 6 which will result in a change of the current delivered by the power supply 8, and the instrument alarm unit 86 will therefore be actuated.

If because of a strong neutron irradiation of the neutron detector the input signal to the first threshold unit 4 exceeds the preset value, this threshold unit will give signals to the actuation circuit 6 as well as to the gate 2 which will no longer pass signals; and this condition of the gate will bring the second threshold unit to give an output signal to the actuation circuit 6 and since this circuit now has received a signal also from the threshold unit 5, it will give a signal to the alarm unit 7, when the first threshold unit 4 delivers a signal for the second time when at the same time the gate 2 will get a signal which opens the gate. Here the attention is directed to FlG. 2, where the abscissa T represents the time and the ordinate the current I. The first horizontal part 91 of the curve represents the check current which is generated by the radioactive source in the neutron detector. The source intensity is selected so that the check current lies between the preset values for the first and second threshold units. At the time t, the neutron detector 1 gets a strong neutron irradiation. The current will then rise rapidly in accordance with the part 92 of the curve. At the time t the actuation circuit 6 receives a signal from the first threshold unit 4 and the gate 2 will no longer pass signals. As a result the current rapidly decreases according to the part 93 of the curve. At the time t the actuation circuit 6 has received a signal from the second threshold unit 5 and gives alarm. At the same time gate 2 opened, which brings about that the current is rapidly increase as indicated by the part 94 of the curve. When the current has reached its maximum, it will start to decrease slowly according to the part 95 of the curve. The speed with which the current decreases is a function of the decay time of the nonstable isotope which has been formed. For supervision of an equipment where a criticality accident might occur, a decay time of 0.5-1 minutes is preferred.

The electronic parts of the system are also shown in FIG. 1. The gate 2 has two make-and-break contacts 21 and 22, which are actuated by the relay winding 23. The amplifier 3 has an electrometer tube 31 in the input stage followed by three transistors 32-34which form a feedback DC amplifier. The amplification factor is controlled by a potentiometer 35. The first threshold unit 4 includes a transistor 48 and a zener diode 47 which in the known way is selected to conduct at a preset voltage. Further on, the unit 4 includes a diode 44, a capacitor 46 and a resistor 45 connected parallel to the capacitor 46. The second threshold unit 5 includes a transistor 54, which is arranged to be nonconducting when the current from the amplifier 3 drops below a preset and for the transistor characteristic value, and to conduct when the current is higher than this value. The actuation circuit 6 is in principle composed of two identical circuits. This doubling of the circuits has been made for the reason that a fault in the auxiliary circuit will not be discovered as easily as a fault in another part of the system. Each circuit has a transistor 62a, 62b; a capacitor 64a, 64b; and a resistor 63a, 63b parallel to the capacitor and a diode 65a, 65b.

The operation of the electronic equipment will be explained, assuming as a starting point that the gate 2 is passing signals and that the signal from the amplifier 3 has a value between the upper limit and the lower limit and that thus zener diode 47 is nonconducting as is the transistor 48 in the first threshold unit 4. The transistor 54 in the second threshold unit 5 is conducting.

If now such a situation occurs that the signal from the amplifier 3 exceeds the upper limit, the zener diode 47 will start to conduct and also the transistor 48 will begin to conduct. The transistor 54 will continue to conduct. This state of the transistor 54 results in a potential on leads 53 and 52, which is output signal from the amplifier 3. As a consequence the zener diode 47 stops conducting and the transistors 48 and 54 will no longer pass current.

During the time when the transistor 48 was conducting, the transistors 62a and 62b received such a potential on their bases that the transistors 62a and 62b started conducting. As already said, the capacitors 64a and 64b had no charge and thus there was no current going to the alarm unit 7 via the leads 61a and 61b and no alarm was given.

When the winding 23 is energized and the signals to the threshold units 4 and 5 are cut off and the transistors 48 and 54 become nonconducting, the following events take place. The current through the winding 23 then ceases and the relay goes to its off position with the contacts 22 and 23 anew in the positions shown in FIG. 1. When the relay is energized and thus the contact 22 is actuated, the capacitors 64a and 64b are charged in such a way that the plate connected to the diode 65a and 65b, respectively, is negative in relation to ground on account of the changed potential on leads 52 and 53.

When the relay goes to its off position and the contacts 21 and 22 assume the positions shown in FIG. 1, the input signal to the amplifier 3 will be reestablished, and the amplifier 3 will deliver an output signal to the threshold units 4 and 5. The further operation depends on the value of the input and output signals when the gate 2 anew is passing the input signal.

Assume in the first place that the input signal has dropped so much that the output signal from the amplifier 3 is not capable of making the zener diode 47 conduct. The transistor 48 will then remain nonconducting and the potential on the lead 42 will have such a value that the transistors 62a and 62b are nonconducting. Although the capacitors 64a and 64b are charged, they cannot be discharged via the transistors 62a and 62b and no current goes to the alarm unit 7 via the leads 61a and 61b, respectively, and no alarm signal is given.

In the second place it is assumed that the input signal has not dropped or at least still has such a value that the output signal from the amplifier 3 is exceeding the upper limit. The zener diode 47 starts conducting and the transistor 48 also starts conducting. However, when the transistor 48 was conducting the first time, capacitor 46 was charged, as has been already stated, and the charge on this capacitor 46 is assumed to have not yet leaked away via the resistor 45. The result is that the circuit which includes the transistor 48 and the relay winding 23 will have such changed working conditions that the current through the relay winding 23 is insufficient to energize the relay. Thus this second time gate 2 will not stop passing the input signal to the amplifier 3, and the output signal from the amplifier 3 will persist and hold the zener diode 47 and the transistors 48 and 54 conducting.

With the transistor 48 conducting and the capacitor 46 charged, the potential on lead 42 is sufficient to make the transistors 62a and 62b conduct. As the capacitors 64a and 64b had been charged with the plate connected to the collector in the PNP-transistor 62a and 62b, respectively, negative with respect to ground the capacitors 64a and 64b will be discharged via the corresponding transistor 62a. 62b, and current goes via the leads 61a, 61b to the alarm unit 7, and this time an alarm is given.

The requisite for an alarm signal is thus that the input signal exceeds a predetermined limit and gives rise to actuation of the gate 2 to stop the input signal followed by a return of the gate 2 to the state where the gate 2 passes the input signal and that then the input signal still has a value above the predetermined limit. This means that the input signal must have a value above the predetermined limit for at least a time period equivalent to the time period required for the sequence described here (1-2 seconds). If the input signal persists above the predetermined limit during a shorter time period, the sequence will take place, as has already been described in the first place, and no alarm signal will be given. This behavior is used to avoid that an alarm signal is given when disturbances of short duration take place and is also made use of for a periodical automatic function test. This function test is performed by feeding short test signal pulses with high amplitude to the amplifier 3 from the test pulse generator 85. if a fault is present in the device, the sequence will not be executed, and this fact can be detected by different methods, i.e., by measuring the total power consumption during such a test. If the current taken from the powersupply assumes a value differing from the normal value, an instrument fault alarm is given.

From FIG. I it is evident that the capacitor 46 as well as the capacitors 64a and 64b are bridged by a resistor 45, 63a and 63b, respectively. In order to obtain the desired function the time constant for each combination of a capacitor and a resistor, that is the combinations 45, 46 and 63a, 64a and 63b, 6412, should exceed the time taken for the described sequence, and it has been found that a time constant of 3-4 seconds is a satisfactory value. if the time constant is too long, there will be a certain dead-time after each sequence.

Modifications, such as the use of an electronic gate 2 instead of a relay, are naturally possible within the scope of the invention as given by the attached claims.

What is claimed is:

1. in a circuit for supervising possible excursions of a signal having a value which normally lies intermediate predetermined upper and lower limits and which provides an alarm when said signal attains a value exceeding said upper limit and which is sustained during a predetermined time period, the combination comprising a power supply unit, an alarm unit, an actuation circuit interposed in the circuit connections between said power supply unit and alarm unit for controlling energization of said alarm unit, first and second threshold detectors connected to said actuation circuit for controlling the same, a gate normally enabled to apply said signal to said threshold detectors when the value of said signal lies between said upper and lower limits, said first threshold detector being responsive to a value of said signal exceeding said upper limit first to initiate a gate-influencing cycle during which said gate is first disabled and again enabled following termination of said time period, and secondly to deliver a signal to said actuation circuit, and thirdly to change its own operational state such that renewed application of a signal at a value exceeding said upper limit will initiate said gate-influencing cycle only if said renewed application takes place following termination of a first predetermined delay, said actuation circuit being normally held by said second threshold detector in a first state where it is not influenced by said signal from said first threshold detector, said actuation circuit being changed over during said gate-influencing cycle to a second state where it closes the circuit between said power supply unit and alarm unit when influenced by a signal from said first threshold detector, said second threshold detector being operative to reconvert said actuation circuit back to said first state following termination of said gate-influencing cycle only after termination of a second predetermined delay.

2. A supervisory circuit as defined in claim 1 wherein said first threshold detector comprises an RC-circuit having a time constant exceeding said time period in order to obtain said first delay, and wherein said actuation circuit also comprises an RC-circuit having a time constant exceeding said time period in order to obtain said second delay.

3. A supervisory circuit as defined in claim 2 wherein said first threshold detector comprises a transistor having a bias determined by a capacitor in said RC-circuit 4. A supervisory circuit as defined in claim 2 wherein said actuation circuit comprises a transistor in a triggering circuit to said alarm, said transistor being influenced by said signal from said first threshold detector and having its current supplied by a capacitor in said RC-circuit 5. A supervisory circuit as defined in claim 1 wherein said gate comprises a relay having a winding energized by a current controlled by said first threshold detector and actuating a first break contact in the signal path to said first and second threshold detectors and a second make contact to change said actuation circuit into said second state.

6. A supervisory circuit as defined in claim 1 and which In- 

1. In a circuit for supervising possible excursions of a signal having a value which normally lies intermediate predetermined upper and lower limits and which provides an alarm when said signal attains a value exceeding said upper limit and which is sustained during a predetermined time period, the combination comprising a power supply unit, an alarm unit, an actuation circuit interposed in the circuit connections between said power supply unit and alarm unit for controlling energization of said alarm unit, first and second threshold detectors connected to said actuation circuit for controlling the same, a gate normally enabled to apply said signal to said threshold detectors when the value of said signal lies between said upper and lower limits, said first threshold detector being responsive to a value of said signal exceeding said upper limit first to initiate a gateinfluencing cycle during which said gate is first disabled and again enabled following termination of said time period, and secondly to deliver a signal to said actuation circuit, and thirdly to change its own operational state such that renewed application of a signal at a value exceeding said upper limit will initiate said gate-influencing cycle only if said renewed application takes place following termination of a first predetermined delay, said actuation circuit being normally held by said second threshold detector in a first state where it is not influenced by said signal from said first threshold detector, said actuation circuit being changed over during said gateinfluencing cycle to a second state where it closes the circuit between said power supply unit and alarm unit when influenced by a signal from said first threshold detector, said second threshold detector being operative to reconvert said actuation circuit back to said first state following termination of said gate-influencing cycle only after termination of a second predetermined delay.
 2. A supervisory circuit as defined in claim 1 wherein said first threshold detector comprises an RC-circuit having a time constant exceeding said time period in order to obtain said first delay, and wherein said actuation circuit also comprises an RC-circuit having a time constant exceeding said time period in order to obtain said second delay.
 3. A supervisory circuit as defined in claim 2 wherein said first threshold detector comprises a transistor having a bias determined by a capacitor in said RC-circuit.
 4. A supervisory circuit as defined in claim 2 wherein said actuation circuit comprises a transistor in a triggering circuit to said alarm, said transistor being influenced by said signal from said first threshold detector and having its current supplied by a capacitor in said RC-circuit.
 5. A supervisory circuit as defined in claim 1 wherein said gate comprises a relay having a winding energized by a current controlled by said first threshold detector and actuating a first break contact in the signal path to said first and second threshold detectors and a second make contact to change said actuation circuit into said second state.
 6. A supervisory circuit as defined in claim 1 and which includes a linear feedback direct current amplifier arranged in the signal path to said first and second threshold detectors.
 7. A supervisory circuit as defined in claim 1 and which further includes an instrumentation alarm unit arranged to be triggered by said second threshold detector when the signal to said second threshold detector falls to a value below said lower limit during the time said gate is enabled. 